1. Field of the Invention
The present general inventive concept relates to an image forming apparatus, and more particularly, to an electrophotographic image forming apparatus and a fusing unit thereof.
2. Description of the Related Art
An image forming apparatus is an apparatus that prints a black and white image or a color image on a printing medium, e.g., printing paper, according to an inputted image signal, and may be, for example, a laser printer, an ink-jet printer, a copying machine, a multi-function printer, a fax machine, etc. An image forming apparatus is classified as an electrophotographic type in which light is irradiated to a photosensitive body to form an electrostatic latent image and a developer adheres to the electrostatic latent image to transfer the same onto a printing medium, or an ink-jet type in which a liquid type ink is ejected onto a surface of a printing medium according to an image signal.
The electrophotographic image forming apparatus is configured such that a surface of a photosensitive body is charged with a predetermined electric potential, a light beam is scanned to the photosensitive body to form an electrostatic latent image due to generation of electric potential difference, and a developer, i.e., a toner powder, adheres to the electrostatic latent image to form a visible image. The visible image formed on the photosensitive body is transferred onto the printing medium, and is fused to the surface of the printing medium.
In order to fuse the visible image formed by the toner powder of the developer to the surface of the printing medium, the electrophotographic image forming apparatus has a fusing unit which applies heat and pressure to the printing medium onto which the visible image has been transferred.
The fusing unit generally includes a heating roller which generates heat of a high temperature, and a press roller which is mounted to closely contact the heating roller. The heating roller includes a heat source such as a lamp, a heating element or the like, an aluminum pipe provided around the heat source, and a release layer provided on the surface of the aluminum pipe. The press roller is provided with an elastic layer on its outer surface, and is in close contact with the heating roller. During the printing operation, the visible image-transferred printing medium receives heat and pressure while passing between the heating roller and the press roller, and the visible image is fused to the surface of the printing medium.
However, in a fusing unit arranged such that the press roller and the heating roller directly contact each other, there is a limitation in a size of a fusing nip formed between the heating roller and the press roller. Because the heating roller is typically made of a metal such as aluminum, although the elastic layer of the press roller is deformed by closely contacting the heating roller, the contact area between two rollers is limited. The large heating area and pressing area in the fusing unit are favorable for the smooth fusing of the visible image. If the size of the fusing nip is small, a fusing efficiency may be decreased. In order to increase the size of the fusing nip, it may be considered to increase diameters of the heating roller and the press roller. This, however, results in the increase in a mounting space of the fusing unit and an increase in an overall size and cost of the image forming apparatus.
To cope with the above problem, a fusing unit capable of increasing a size of a fusing nip using a heating belt has been developed and used. The fusing unit using the heating belt is arranged such that the heating belt contacts the press roller while circulating by being supported on plural rollers. The rollers supporting the heating belt include a heating roller which heats the heating belt, and a driving roller which rotates by receiving a driving force. The driving roller has an elastic layer, and is mounted to closely contact the press roller. Thus, if the driving roller and the press roller closely contact each other, the elastic layers of the respective rollers are deformed, and the contact area therebetween is increased. The fusing nip between the heating belt and the press roller is also increased, thereby improving the fusing efficiency.
The so-called belt type fusing unit should have a tension adjusting device to maintain a constant tension of the heating belt, which is supported on the plural rollers. This is to enable the heating belt to smoothly circulate without slipping on the driving roller.
The conventional belt type fusing unit uses a tension roller which presses on one side of the belt, similarly to a common belt device, to maintain the constant tension of the heating belt. The tension roller is mounted such that a pressing force is changed. In order to prevent the contamination of the tension roller due to foreign substances adhering to the heating belt, a cleaning roller is mounted to contact the heating belt to remove foreign substances from the heating belt. However, if the number of rollers rotating while contacting the heating belt is increased, a resistance to the circulation of the heating belt is also increased. Further, the installation of the expensive tension roller and cleaning roller increases a manufacturing cost, and the increased number of rollers causes a high frequency of operational trouble.